The present invention relates to solar filters of the type employed on optical instruments, such as telescopes, to filter sunlight. Such filters are required to protect the user's eyes from harm as well as to prevent overheating of the optical instrument and to enable viewing and photography through the instrument under sunlight conditions.
Solar filters essentially require a metal component to reflect or otherwise block solar radiation, particularly infrared radiation and ultraviolet radiation. Traditionally, the silver particles in exposed black and white photographic film have been employed as effective safety filters for sunlight. More recently, metalized plastic films, such as polyester films having a fiber deposited aluminum coating thereon have been employed for the purpose.
Generally such filters are disc shaped and are mounted externally of the telescope or other optical instrument to cover the lens. The metalized filter element passes a small percentage of the total sunlight therethrough and effectively blocks most of it from entry into the optical instrument.
One difficulty with prior art filters is that, if light conditions change or if it is desired to change the brightness of the image provided by the optical instrument, the filter provides no adjusting or compensating ability other than, possibly, replacing one filter with another of different light passing capability. This is obviously impractical and would require a stockpiling of numerous filters of different density and light passing ability for each different sized optical instrument. Further, fine gradations of density of solar filters are simply not available and so adjustment can only be made with large incremental changes in the amount of light allowed to pass into the optical system of the instrument.
It is accordingly an object of the present invention to provide a novel solar filter structure which is adjustable to admit, in infinitesimally small increments, varying quantities of light through the filter to adjust brightness of the image and/or compensate for changing light conditions.
It is another object of the present invention to provide such a solar filter in which such adjustments may be made without changing the effective optical aperture provided by the filter.
It is a further object of the present invention to permit mounting in a simple manner that permits easy removal and allows ventilation, thereby avoiding heat buildup.
Other objects and advantages of the present invention will be apparent from the following description thereof.